1. Field of the Invention
This invention relates generally to the field of integrated circuits. In particular, the invention relates to achieving accurate temperature-invariant sampling frequencies in an electronic device, such as, a multiple message non-volatile multilevel analog signal recording and playback system.
2. Description of Related Art
In a Sample Data System, there is always the need to band limit the incoming analog signal and sample it periodically. Band-limiting of the analog signal is performed through a suitable lowpass or a bandpass filter, and the periodic sampling is performed by sampling circuitry with an accurate periodic time-base. The periodic time base is generated from a suitable oscillator.
One form of monolithic implementation of a suitable filter is the continuous time active MOSFET-RC type. In a MOSFET-RC filter, MOSFET transistors are used in the non-saturation region with a control voltage on the gate to generate an effective resistance. Processing tolerances of integrated MOSFET transistor and capacitor parameters may create +xe2x88x920.50% variation in the filter time constants. To maintain tight tolerances on the filter time constants, it becomes necessary to have a control loop, referenced to a stable reference, to control the time constants of the filter over process and ambient changes. In implementations where the time-base oscillator is also specified to be implemented in a monolithic form with the filter, the same process tolerances will create +xe2x88x920.50% variations on the oscillator frequency. Also, in order to achieve an accurate and stable oscillation frequency, the oscillator needs to be referenced to a stable and accurate reference as part of the control loop, as well. Various systems and methods have been devised to accomplish this.
One such prior attempt, detailed in U.S. Pat. No. 5,352,934 issued to Khan, describes an integrated circuit system wherein both filter time constants and oscillator frequencies each utilize a suitable reference. Both the filter and the oscillator are referenced to common reference circuitry through a control loop. FIG. 1 shows a circuit, as implemented in Khan, for achieving variable sampling frequencies, in which, the filter cutoff frequency tracks the oscillator frequency. As will be discussed this approach has certain disadvantages.
As shown in FIG. 1, the current Iosc drives the oscillator 1. A current Ix, derived and related to Iosc, is used to produce a control voltage VCNTRL for the MOS resistor used in the MOSFET-RC filter 2. Thus, as the oscillator current (Iosc) is changed (e.g. as per change in sampling frequency), the gate voltage to the MOS devices is changed which modulates the resistance of these MOS devices which in turn tunes the cutoff of the continuous time MOSFET-RC filter 2. The conversion from the input current to the oscillator frequency and the conversion of the input current to the cut-off frequency includes numerous discrete components. Unfortunately, each component will have a certain process spread and this will cause undesirable variations in both the oscillator frequency and the cut-off frequency.
The oscillator current (Iosc), in the prior circuit of Kahn, is generated by a positive temperature coefficient (TC) current (denoted PTAT) from a proportional to absolute (PTAT) current generator 3 and a negative TC current (denoted NTC) using a negative TC generator 4. Both these currents are trimmable (i.e. they can be changed on chip by means of digital control signals derived from flash cells (e.g. storage cells)) within the temperature independent current generator 5. These trimmable positive TC currents and negative TC currents are fed into a tuning network to produce a stable oscillator current (Iosc) for the oscillator 1 and the related current Ix for the filter. The tuning network 6 sums the trimmed positive TC currents and negative TC currents together and adjusts the magnitudes of the currents also using trim bits.
Unfortunately, there is extensive mirroring of the currents involved in the above method. Every current mirror in the system introduces an error in the output current due to transistor mismatch. Thus, the more current mirrors that are used in the system, the wider the spread of the output currents from the positive and negative TC generators 3 and 4. Therefore, the more current mirrors that are used, the wider the distribution of the oscillator current (Iosc). Also, to achieve the desired sampling frequencies, the Iosc current is trimmed using trim bits in the tuning network 6 to achieve a proper Ix for the desired sampling frequency. Unfortunately, this means the oscillator frequency needs to be linear over a large frequency range. It should be noted, that the storage cells 7 (e.g. flash cells) are programmable such that they can output a logic one or a logic zero so that they can be used as digital control signals to fine-tune (or TRIM) the outputs of the respective blocks of the integrated circuit. This optimizes the integrated circuit""s performance after manufacture without making a mask change. An example of the use of trim bits can be seen in U.S. Pat. No. 5,933,370 issued to Holzmann, et al.
Thus, the prior circuit of Khan, as illustrated in FIG. 1, suffers from certain disadvantages. As previously discussed, to achieve the different sampling frequencies, the oscillator frequency has to be varied. In order to achieve a stable oscillator frequency, the current driving the oscillator Iosc needs to be accurate. However, extensive mirroring is required to generate the oscillator current Iosc. Due to this extensive mirroring, even a slight mismatch between devices can create large errors in the oscillator current Iosc. This can cause the oscillator frequency to be inaccurate and can result in the filter cutoff, which is based on Ix in turn derived from Iosc, to vary drastically which can result in a poor Signal to Noise plus Distortion ratio (SINAD) and sound quality. Further, to achieve the different sampling frequencies the oscillator current Iosc has to be trimmed to achieve the proper Ix current and the right oscillator frequency, which means that the oscillator needs to operate linearly over a large frequency range. Also, the oscillator duty cycle can vary significantly due to mismatch between the devices.
Therefore, there is a need in the art for generating a stable oscillator current, utilizing a minimal amount of mirroring action, which can be used directly to generate the oscillator frequency.
An apparatus and method for achieving accurate temperature-invariant sampling frequencies in a device, such as, a multiple message non-volatile multilevel analog signal recording and playback system is described. An oscillator is used to generate an oscillation frequency. A bandgap voltage generator generates a zero temperature coefficient voltage reference (V(OTC)) that is independent of temperature. This V(OTC) is applied to the oscillator. A variable temperature coefficient voltage (V(TC)) that compensates for temperature coefficient variations of a resistor to which V(TC) is applied produces a stable oscillator current Iosc. Therefore, the stable oscillator current Iosc is likewise independent of the temperature coefficient variations of the resistor. The stable oscillator current Iosc is applied to the oscillator such that the oscillator generates a stable temperature-invariant oscillation frequency. A digital divider is used to digitally divide the stable oscillation frequency by a predetermined amount to produce an accurate temperature-invariant sampling frequency. A filter is then used to filter incoming signals utilizing a filter cutoff that tracks the sampling frequency.
In one embodiment of the present invention, the stable temperature-invariant oscillator frequency is a fixed frequency and a plurality of different sampling frequencies can be derived from the fixed oscillator frequency by simply digitally dividing the fixed oscillator frequency down to the desired accurate temperature-invariant sampling frequency. Because the oscillator is run at a fixed frequency, the oscillator can be optimized for a particular fixed frequency thus reducing the linearity requirement of the oscillator frequency over a large frequency range.
Advantageously, the stable temperature-invariant oscillator current that directly drives the oscillator is created utilizing a minimal amount of mirroring action such that variations in the oscillator current are minimized. Further, the oscillator current is used to directly generate the oscillator frequency which is digitally divided down into the desired accurate temperature-invariant sampling frequency thereby further eliminating errors due to component process variations. Also, in one embodiment, divider control bits are used to digitally divide the fixed oscillator frequency down to the desired sampling frequency. Additionally, since the oscillator frequency is digitally divided down to the desired sampling frequency, the requirement that the duty cycle of the oscillator be accurate is lessened.
Other features and advantages of the present invention will be set forth in part in the description which follows and the accompanying drawings, wherein the preferred embodiments of the present invention are described and shown, and in part will become apparent to those skilled in art upon examination of the following detailed description taken in conjunction with the accompanying drawings, or may be learned by the practice of the present invention. The advantages of the present invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.